sharing a mailbox via IMAP - possible?

I'd like to be able to share mailboxes between users. I fully understand that the web client doesn't do this yet, but as we're using Thunderbird at present, that's not an issue. There is some pretty good sharing info at Zmmailbox - ZimbraWiki and while it doesn't list mailboxes as something that can be shared, the folder methodology would seem to support it.

I followed the sequence that was presented for sharing a calendar and it seemed to work--i.e. the folder is available in the second account)--but nothing is ever visible in the folder. Is this just not possible period? Or is there some way of doing this that I am not grasping?

I have searched high and low with the various tools and can't find anything that doesn't specifically apply to the web client. Given the fabulous REST interfaces and whatnot, it seems that more advanced functionality is available outside the web client. I'm hoping that's the case here.

Yeah, bug 6937 is already partially implemented for 5.0. Both mountpoints and Other Users' NAMESPACE will work correctly in 5.0 provided that the other user is homed on the same ZCS server as you. Full cross-server IMAP mailbox sharing may have to wait for 5.5.